1. Field of the Invention
This invention relates to thermosetting polyester compositions and processes for making molded parts from such compositions. In particular, the present invention relates to the fabrication of molded parts using chemically thickened thermosetting compositions.
2. Description of Related Art
Composite materials are known to have the advantages of high strength, light weight, design flexibility, dimensional stability, corrosion resistance, parts consolidation, better finishing, and low tooling cost over traditional construction materials such as metal, ceramics, and wood. Fiber-reinforced thermosetting polyester composites are widely used in many applications, e.g. marine, automotive, transportation, electrical, construction, consumer and industrial goods, etc. Compared to the composites made from other types of thermosetting resins such as vinyl ester, epoxy, and polyamide, thermosetting polyester composites have the advantages of lower material cost and easy material handling during processing. Therefore, unsaturated polyester resins are the materials of choice for most of the fiber-reinforced thermosetting composites in applications in which the working environment of the composite is not very harsh.
Fiber-reinforced thermosetting polyester composites usually consist of reinforcing (or reinforcement) fibers, either in chopped or continuous form, embedded in a matrix of one or more unsaturated polyester resins. The unsaturated polyester resins are made from a reaction of one or more glycols with an unsaturated dicarboxylic acid or its anhydride, or with a mixture of the unsaturated acid or its anhydride with a saturated dicarboxylic acid or its anhydride. The molecular weight of the polyester is controlled through any conventional means.
In the formation of the matrix, the unsaturated polyester resin is blended typically with (1) one or more monomers capable of crosslinking with the vinyl groups in the polyester, (2) one or more polymerization initiators, (3) promoters for use in combination with the initiator, and (4) various other additives which impart desired characteristics to the matrix upon cure or which will improve the processing and/or curing properties of resin. Such compositions are more fully described in the technical literature, such as in U.S. Pat. Nos. 5,589,538; 5,202,366; 5,516,819; and 4,535,110, which are incorporated herein by reference.
When molding compositions are to be used as sheet molding compositions ("SMCs"), thickening agents are used. Thickening agents increase the viscosity of the molding composition, so that the thickened system is relatively tack free and easy to handle for placing the molding composition in the mold. The thickened composition's high viscosity also helps carry reinforcement fibers to the extremities of the mold when compression is applied to the mold.
Thickening is typically achieved by adding an alkaline material, such as magnesium oxide or hydroxide, to the uncured molding composition. The alkaline material interacts with residual acidity in the polyester resin component to build viscosity. The kinetics of this process are rather slow, requiring several days to achieve a viscosity appropriate for sheet molding operations, which is why the thickening process has been referred to as "maturation". Heat cannot be used to speed up the kinetics, since that would result in premature crosslinking of the molding composition.
A slow maturation process slows down production, requiring more factory space and/or equipment to carry out a given rate of production. It also allows time for premature phase separation to occur between components of the molding composition that are not completely compatible with each other, such as phase separation between the polyester resin(s) solubilized in the monomer and the thermoplastic low profile additives ("LPAs"). Such phase separation cannot be prevented in SMCs by agitation during maturation, because of the presence of the reinforcing fibers. This has either limited the range of component types and amounts that are added to SMCs or has required the addition of compatibilizers, the presence of the latter often having an adverse effect on the physical properties of the resulting molded part.
To achieve a higher maturation rate, the concentration of thickening agent has typically been increased beyond that required to achieve the desired viscosity endpoint, together with providing a sufficient amount of acid end groups on the polyester resin component. A further acceleration of maturation is typically achieved by adding water, which facilitates the thickening reaction. The downside of this approach is that maturation can, and generally will, continue irreversibly to a point at which the viscosity of the molding composition becomes undesirably high. Molding compositions with such high viscosity require high molding pressures, which puts high stresses on the mold. High mold stresses have until now required molds to be made of high strength materials, such as hardened steel, (to avoid deformation of, or damage to, the mold and to reduce wear from repeated use) and also required equipment that can apply high pressure, increasing the capital investment required to make molded parts.
This increase in viscosity over time creates a specific time window within which the molding composition is at a desired viscosity. The more thickening agent is added to speed up maturation, the narrower the time window for the desired viscosity. A narrow time window requires the fabricator to have equipment able to handle significant variations in viscosity from sheet to sheet of SMC due to greater difficulty in accurately predicting when each sheet is ready for molding and the variations in timing between when the thickening agent is added and the part is molded inherent in a typical factory environment.
A factor which has been found to cause significant variability in maturation rate is the presence of varying amounts of water as a contaminant. Water is naturally present in hygroscopic materials such as fillers and may be introduced in trace quantities by the polyester itself, since water is a byproduct of making polyester, and by contact of ingredients with humidity in the atmosphere. Precise control over the amount of water contamination in SMCs is difficult and to some extent impractical. Therefore, there is a degree of unpredictability/variability in the maturation rate from sheet to sheet even if the timing for each sheet were kept the same.
Due to the above variability factors, the fabricator of molded articles has had to use molds and molding equipment with excess capacity to handle potential stress of handling SMCs over a wide range of viscosities ranging far into the high end, where molding is difficult.
There is also a greater risk of scrap if there is down time on the production line, since there is no known way to reverse the maturation process once it has gone too far, and even with higher amounts of thickening agent, the initial maturation rate is generally still too slow to prevent premature phase separation of incompatible or noncompatibilized components.
There is thus a desire to achieve a high rate of viscosity increase in the beginning of maturation followed by a relatively long time window in which viscosity remains within a desired range for fabrication into the molded part.
There is also the desire for molding compositions which can incorporate components that tend to form a separate phase in the uncured SMC.
There is especially a desire in the art for SMCs that predictably reach a viscosity plateau at lower viscosities, because such SMCs would lend themselves to use with less expensive, easier to fabricate molds and molding under lower temperature and pressure conditions.
These and other objectives are surprisingly achieved by the present invention.